1. Field of the Invention
This invention relates to a charger for charging a surface of an image receptor such as a photoconductor drum on which a toner image is formed at a uniform potential in an image formation apparatus such as an electrophotographic copier or a laser beam printer and in particular to improvement to hold the surface of the image receptor and the surface of the charger in a clean condition over a long term.
2. Description of the Related Art
In an electrophotographic copier or a laser beamprinter, a surface of an image receptor such as a photoconductor drum is charged uniformly and then is exposed to light in response to image information for forming an electrostatic latent image and further the electrostatic latent image is developed in toner to form a toner image of a visible image and last the toner image is transferred to an image receptor such as an intermediate transfer body or a record sheet to form a record image. As a charger for uniformly charging the surface of the photoconductor drum, hitherto a corona discharge unit of non-contact type has been frequently used, but ozone produced by corona discharge easily has an adverse effect on the environment and human bodies and thus in recent years, replacement with a charger brought into direct contact with an image receptor for use has been rapidly increased.
Hitherto, as this kind of contact-type charger, a charge roll wherein surroundings of a conductive axis core member are covered with conductive rubber and the conductive rubber is brought into contact with the surface of a photoconductor drum by applying predetermined contact pressure has been known. In the described charge roll, a minute gap is formed between the conductive rubber and the photoconductor drum and charges are given to the photoconductor drum by discharge occurring in the minute gap. However, in the charge roll, the conductive rubber layer becomes deformed under the contact pressure and thus it is hard to bring the conductive rubber into uniform contact with the photoconductor drum and there is the disadvantage that charge unevenness develops on the photoconductor drum because of nonuniformity of contact.
Then, JP-A-4-232977, JP-A-5-72869, etc., discloses a charger having a configuration wherein a rotation tube formed like a cylinder is brought into contact with a photoconductor drum. Specifically, the rotation tube having a predetermined resistance value is brought into contact with the photoconductor drum and a conductive axis core member for supplying power to the rotation tube is placed in the rotation tube so as to pierce the rotation tube and a DC bias voltage is applied to the axis core member. According to such a configuration, the rotation tube is flexibly adapted to the outer peripheral surface of the photoconductor drum and further the rotation tube itself is electrostatically attracted onto the outer peripheral surface of the photoconductor drum as the bias voltage is supplied, so that the rotation tube and the photoconductor drum can be brought into uniform contact with each other and it is made possible to evenly charge the photoconductor drum as compared with the related art wherein the conductive rubber layer is pressed against the surface of the photoconductor drum.
A charger disclosed in JP-A-11-125956 comprises a conductive foam layer placed surrounding a conductive axis core member and a rotation tube bonded to the surroundings of the foam layer; the hardness of the foam layer is set extremely soft, whereby the rotation tube comes in contact with a photoconductor drum with flexibility. Therefore, in the charger disclosed in the gazette, the photoconductor drum can also be evenly charged by applying a bias voltage of a predetermined magnitude to the axis core member.
By the way, in an electrophotographic copier, etc., using this kind of charger, an electrostatic latent image formed on the surface of a photoconductor drum is developed in toner and the toner image is transferred to an image receptor and thus remaining toner untransferred onto the photoconductor drum, paper powder, etc., is easily deposited on the surface of the photoconductor drum. Fine particles of silica, alumina, etc., are added to toner to adjust the frictional charge amount and fluidity. When a toner image is formed on the photoconductor drum, the external additives are deposited on the photoconductor drum. Further, if a dual-component developer with toner and carrier mixed is used as a developer of an electrostatic latent image, the carrier may be deposited on the surface of the photoconductor drum.
Thus, the foreign materials are easily transferred from the photoconductor drum to the surface of the charger coming in direct contact with the photoconductor drum and if toner, etc., is deposited on the surface of the charger, discharge becomes abnormal only in the deposition part and the surface potential of the photoconductor drum tends to become high. If a foreign material of a comparatively large particle diameter such as carrier is deposited, a charge failure of the photoconductor drum occurs. That is, unevenness occurs in the surface potential of the photoconductor drum and a difference appears in the development amount and thus inconsistencies in density become conspicuous particularly if an attempt is made to develop a halftone image.
The external additive is extremely small as compared with toner and carrier. Thus, if the external additive is deposited on the surface of the charger to an extent, it does not much affect the charge performance itself. However, if the charger is repeatedly rotated with both the external additive and toner deposited on the surface of the charger, the toner tends to be fixedly deposited on the surface of the charger with the external additive as a core. The fixedly deposited toner is transferred to the surface of the photoconductor drum like stripes and at last is spread on the surface of the photoconductor drum like a thin film. Such fixed depositing of toner on the charger and the photoconductor drum is called so-called filming phenomenon. If the filming phenomenon occurs, inconsistencies in density or stripes occur in a formed image and the image quality is remarkably degraded.
To circumvent occurrence of such a problem, a copier, a printer, etc., in related arts generally comprises a cleaner for cleaning the surface of a photoconductor drum placed between a toner image transfer section and a charger, whereby foreign materials of toner, carrier, etc., deposited on the photoconductor drum are collected before the charger. However, the cleaner also scrapes off the photosensitive layer of the surface of the photoconductor drum at the same time as it removes the foreign materials of toner, etc., from the surface of the photoconductor drum and thus there is a problem of shortening the life of the photoconductor drum. This problem is noticeable particularly in a cleaner of the type wherein a cleaning blade is pressed against the surface of the photoconductor drum. When a cleaner is provided for the photoconductor drum, if the capacity of a collection box of waste toner collected by the cleaner is set large, the image formation apparatus itself is upsized; a large detriment is produced particularly in a so-called tandem full-color copier having a photoconductor drum for each of color toners of yellow, magenta, cyan, and black. Particularly, in recent years, a photoconductor drum of a small diameter has been adopted for the purpose of miniaturizing a full-color laser beam printer and the space surrounding the photoconductor drum has been remarkably lessened. Improvement in toner to enhance the transfer efficiency of a toner image is also advanced. For example, spherical toner described later is the case; this toner is hard to be removed from a photoconductor drum if a cleaning blade is simply provided. Therefore, needs for a so-called cleanerless structure with no cleaner provided for a photoconductor drum grow, and the deposition of toner on the charger, the occurrence of the filming phenomenon described above is an extremely important problem to be solved for providing a cleanerless image formation apparatus.
It is therefore an object of the invention to provide a charger for making it possible to prevent foreign materials of toner, carrier, etc., from being deposited on the charger itself and a filming phenomenon from occurring on an image receptor such as a photoconductor drum for stably charging the surface of the image receptor over a long term.
That is, the invention provides a charger for uniformly charging a surface of an image receptor comprising a cylindrical rotation tube coming in contact with the image receptor on which a toner image is formed, the rotation tube to which a predetermined charge bias is applied and a drive section adapted to drive the rotation tube at a predetermined peripheral speed, wherein the following relation is satisfied:
1.01 less than (V2/V1)xe2x89xa61.10 
where V1 is a peripheral speed of the image receptor and V2 is a peripheral speed of said rotation tube.
Foreign materials of toner and carrier deposited on the charger originally are deposited on the image receptor such as a photoconductor drum when an electrostatic latent image is developed; the foreign materials are transferred to the charger in the contact area between the charger and the image receptor. Thus, if a speed difference is provided between the image receptor and the rotation tube of the charger rotating while they come in contact with each other, a speed difference exists between the foreign material proceeding while being deposited on the image receptor and the rotation tube, and thus the foreign materials become hard to be deposited on the rotation tube. If the carrier, etc., is deposited on the rotation tube, the part where the carrier, etc., is deposited again passes through the contact area with the image receptor, whereby the deposited foreign materials are scraped off the surface of the rotation tube and continuous depositing of the foreign materials on the rotation tube can be prevented as much as possible. The inventors conducted an experiment and acknowledged that if the speed ratio of the peripheral speed of the rotation tube V2 to the peripheral speed of the image receptor V1 is larger than 1.01, depositing of foreign materials on the surface of the rotation tube can be suppressed and a good record image with no spot-like image dirt can be provided.
On the other hand, if the speed difference therebetween is too large, a large shear stress acts on the toner sandwiched between the image receptor and the rotation tube and thus the toner is easily fixedly deposited on the surfaces of the rotation tube and the image receptor. The inventors also conducted an experiment and acknowledged that if the speed ratio of the peripheral speed of the rotation tube V2 to the peripheral speed of the image receptor V1 is 1.10 or less, depositing of toner on the surfaces of the image receptor and the rotation tube, namely, occurrence of a filming phenomenon can be suppressed and a good record image with no image quality unevenness can be formed.
The invention provides a charger rotating while coming in contact with an image receptor on which a toner image is formed, the charger for uniformly charging a surface of the image receptor, comprising a cylindrical rotation body coming in contact with the image receptor, the rotation body to which a predetermined charge bias is applied and an axis core member piercing the rotation body, wherein the following relation is satisfied:
RZ/Smxe2x89xa61xc3x9710xe2x88x922 
where a surface roughness of the rotation body is RZ (xcexcm) and an average spacing between concavity and convexity is Sm (xcexcm).2 
Furthermore, the invention provides a charger rotating while coming in contact with an image receptor on which a toner image is formed, the charger for uniformly charging a surface of the image receptor, comprising a cylindrical rotation tube coming in contact with the image receptor, the rotation tube to which a predetermined charge bias is applied, an axis core member piercing the rotation tube, and an elastic body placed surrounding the axis core member for pressing the rotation tube against the image receptor, wherein the following relation is satisfied:
RZ/Smxe2x89xa61xc3x9710xe2x88x922 
where a surface roughness of the rotation tube is RZ (xcexcm) and an average spacing between concavity and convexity is Sm (xcexcm)